Alex Bosworth has a good post over on his Weblog on 10 Places You Must Use Ajax. This is a nice, thoughtful list, and it's apparent Alex has a good visceral grasp of not just AJAX, but of interactive Web site or Web application design -- it's a great idea to highlight the areas where the unique qualities of AJAX techniques serve a Web page or Web app in best stead.
While the title might be a bit strongly worded ("must" sounds pretty obligatory), in some ways this is much more helpful than the initial inclination you might have to give a list of what's wrong in AJAX development. (Although he has actually also compiled a nice list of exactly that, Ajax Mistakes.)
It's great to know what pitfalls to avoid, but AJAX development seems to be attracting large numbers of delevopers from a traditional applications development background. A lot of these guys don't have the intuitive understanding of what AJAX means for Web development that an old-school Web dev might have. Telling these guys where best to apply the new AJAX fu might give them a real leg up on exploiting the strength of the Web medium.
This is particularly important given the common misperception that AJAX means making your browser-based app act like a desktop app. Your AJAX Web app may be way more responsive and interactive than a traditional Web app, but there are still really important differences between the two.
Alex's post is good place for these guys to start -- before they roll up their sleeves and get their hands dirty with XMLHttpRequest, or some toolkit that wraps that stuff up for them.